This invention relates to anchor bolts which are installed in mine roofs for support purposes.
In the installation of mine roof bolts, such as point anchor resin bolts, a resin capsule is inserted into a vertical bore in the mine roof and the bolt is inserted into the bore for pushing the capsule to the back of the bore. Continued upward pressure by the bolt fractures the capsule and the bolt is rotated to mix the resin components. The resin is then permitted to harden to secure the bolt in the bore. After the resin hardens, it is necessary that the roof support plate associated with the bolt assembly be moved into supporting engagement with the mine roof.
Various means have been devised to accomplish this purpose by providing structural parts on the roof bolt which are adapted to break away when sufficient force is applied to the bolt head, thereby permitting the roof bolt and support plate to move upwardly into engagement with the mine roof. Illustrative of such structures are those disclosed in U.S. Pat. Nos. 3,979,918 and 4,023,373. Although these prior devices all are concerned with the present problem, they do not accomplish this purpose in the simple and inexpensive manner necessary to render the manufacture thereof economically feasible nor can they be produced with such consistency that they can break away at a precise torque.
It should further be noted that many roof bolts are ineffective because they are not installed properly, the resin is not mixed completely, or the head of the bolt seizes on the plate, and the torque is lost overcoming this friction. Additionally, the operators of machines used to install roof bolts are commonly unaware that their machines have lost the power to tighten bolts to the desired torque. This is due to reduced voltage in the power supply, wear on the machines, and inaccurate and erroneous settings of hydraulic pressures. Also, the friction between bolt heads and plates varies widely due to varying thrust of the bolting machine.